The composite video signal utilized by a television receiver contains horizontal and vertical synchronizing information which is used by receiver synchronizing circuitry to provide accurately timed scanning of the kinescope electron beams from line to line and frame to frame. In the NTSC standard, a transmitted television signal is composed of a radio frequency (rf) carrier signal which is modulated with the composite video signal to form an amplitude modulated ac signal with the composite video signal defining the envelope of the modulated rf carrier. This modulated rf carrier signal is applied to tuner and intermediate frequency (IF) circuitry, producing a heterodyned output signal which is demodulated by a video detector which reproduces the composite video signal.
The input to the video detector is an amplitude modulated signal, so it is possible to reproduce the composite video signal by detecting either the upper envelope portion having positive-going sync pulses with the video black level positive with respect to the video white level or the lower envelope portion of the modulated signal having negative-going sync pulses with the video black level negative with respect to the video white level. The choice as to whether the upper "positive" or lower "negative" polarity envelope portion is selected may be determined by the choice of circuit components or processing techniques, some of which may operate more efficiently or technically superior with respect to one polarity signal over the other.
The demodulated composite video signal is applied to a synchronizing pulse (sync) stripper, which produces an output in response to the high amplitude (either positive-going or negative-going) horizontal and vertical sync pulses, hereinafter referred to as positive and negative polarity pulses regardless of the actual voltage levels of the pulses themselves. This composite sync signal is processed to separate the vertical and horizontal sync pulses from each other by differentiating the signal to reproduce the short duration horizontal sync pulses and integrating the signal to reproduce the long duration vertical sync pulses. The choice of sync separator components will of course be determined by the polarity of the composite sync signal. For example, for positive and negative polarity sync pulses, the vertical sync pulses will be reproduced by integrating the composite sync signal positively or negatively toward a threshold level, respectively. The threshold detection circuits must be designed to establish the proper threshold level and polarity.
It is desirable to provide many television receiver functions by forming the appropriate circuits as parts of integrated circuits, in order to reduce receiver size, weight and cost. For example, many receivers utilize an integrated circuit to provide accurately timed horizontal and vertical deflection drive pulses which are applied to the appropriate horizontal and vertical deflection circuits. These integrated circuits may include one or more frequency and phase control loops to provide extremely precise deflection timing. Since the horizontal and vertical deflection timing must be closely controlled with respect to the occurrence of horizontal and vertical sync pulses, the previously described deflection integrated circuits may also include a sync pulse processor which receives the differentiated and integrated composite sync signal from a sync separator. The sync pulse processor then produces horizontal and vertical sync pulses of a single predetermined polarity which are used to control other functions on the integrated circuit. Primarily because of the size of the capacitors in the differentiating and integrating circuits, it may not be practicable to form the sync separator circuitry as a part of an integrated circuit. It may be more efficient to provide the sync separator within the receiver as a discrete circuit having discrete components.
As discussed, the output from the sync separator may represent either positive or negative polarity sync pulses. In order to provide universal application to any receiver, the previously described deflection integrated circuit must include a sync processor which can accept differentiated and integrated sync signals of either positive or negative polarity. It is also desirable to eliminate any requirement for additional terminals or pins on the integrated circuit which might be used to select the sync pulse polarity for a particular receiver application in order to avoid an undesirable increase in the number of integrated circuit pins.